arm64: dts: salvator-x: add Salvator-X board on R8A7796 SoC

This patch adds initial board support for R8A7796 Salvator-X.

Signed-off-by: Takeshi Kihara <takeshi.kihara.df@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>